1. Field Of The Invention
A considerable problem in connection with the detection of surface imperfections using, for example, eddy current technique, is the distance dependence (the so-called lift-off dependence) between the transducer and the object under test, which often causes considerable disturbance of the measurements.
2. Prior Art
One method of suppressing the LO dependence is disclosed in Swedish Pat. No. 7507857-6, including the patent of addition.
A similar method is described by Hugo L Libby in "Introduction to Electromagnetic Nondestructive Test Methods". The method of Libby can be regarded as a form of vector transformation technique based on phase discrimination, which means that vectors with different directions are separated with the aid of phase-sensitive/phase-controlled rectifiers.
A greatly limiting factor in Libby's method, especially in the practical case involving superficially located defects, for example surface cracks, is that the directions of the fault vectors in the impedance plane (FIG. 3) substantially coincide with the direction/orientation of the LO vectors (.DELTA.LO) (i.e. .alpha. is small), thus making it very difficult to efficiently separate the fault vectors (F) from the LO vectors by using phase discrimination only, and simultaneously suppress the lift-off dependence.
For, for example, small/shallow surface cracks the angle (.alpha.) between the fault direction and the LO direction may only be 10.degree.-15.degree. in case of a hot (1000.degree.) material.